Abstract Clostridium difficile is a Gram-positive, spore-forming bacteria that causes severe diarrhea and C. difficile infection (CDI) ranks fourth among the highest number of infections in hospitals. Annually, CDI causes as many as 500,000 infections and 15,000 deaths in the USA and costs up to $4.8 billion. Development of effective vaccines is important for prevention and treatment of CDI. There are C. difficile vaccine candidates that are comprised of inactivated toxins A and B (called toxoids) and none have demonstrated significant clinical efficacy in CDI prevention. We propose to use our novel vaccine delivery platform that presents antigens on the surface of nanoparticles derived from recombinant outer membrane vesicles (rOMVs) of probiotic E. coli strains. rOMVs represent a promising, low-cost vaccine delivery method with improved features over conventional vaccines including better antigen presentation, technical readiness, simplicity, thermostability and flexibility of the manufacturing process. The OMV-based vaccine platform has been validated clinically by an approved vaccine (Bexsero, GSK) that is efficacious in prevention of group B meningococcal infections. Versatope?s scientific founders have demonstrated that our rOMV-based vaccines containing polypeptide antigens and poly-N-acetylglucosamine (PNAG, a conserved bacterial surface polysaccharide) are effective in prevention of bacterial and viral infections in animal models. In this study, we hypothesize that C. difficile toxoids A and B, and PNAG presented on the surface of E. coli rOMV will be displayed in a three- dimensional antigenic complex to generate strong and long-lasting immunity responses that may result in a better protection against CDI than the conventional vaccines composing purified toxoids formulated with the Alum adjuvant. The primary objective of this proposal is to select a novel C difficile vaccine candidate(s) for further evaluation and development.